“Microcreping”, sometimes called “Dry Microcreping,” refers to longitudinal treatment of traveling flexible sheet materials under substantially dry conditions in which a drive force is produced by pressing the sheet material against a drive roll. This positively propels the material through a confined retarding passage, with microcreping action on the sheet material occurring in the transition between driving and retarding regions. Because such microcreping does not depend upon adhesion of the sheet material to the drive surface or a wet condition of the material, a particularly wide range of properties is obtainable. (Note: The dry microcreping here described must not be confused with wet creping or creping based on adhesion, performed for instance on a Yankee Dryer. There have been instances in which such processes too have been referred to as “microcreping”, though they are completely different, incapable of the results achievable with “dry microcreping”.)
“One roll microcreping”, i.e. one roll dry microcreping, refers to microcreping that relies upon a single drive roll having a surface capable of mechanically gripping the inner face of the sheet material. A running length of the sheet material is pressed with considerable force face-wise against this moving surface by a stationary pressing member whose face is freely slippable (i.e. smoothly, continuously slippable) relative to the outward face of the material which it engages. Because of the variable geometry of the treatment region made possible with such an arrangement, a particularly wide range of treatments is possible.
A “bladed microcreper” or dry microcreper refers to a one roll microcreper in which retarding is dependent upon extrusion of the treated material between opposed retarder surfaces, the retarder on the roll side being of blade form.
A “bladeless microcreper” or dry microcreper refers to a one roll-microcreper that does not have such a blade.
Depending upon the nature of the flexible sheet material and the conditions of treatment, by microcreping with a one roll microcreper: individual fibers of a sheet material can be crimped while remaining an integral part of the sheet; minute crepes or coarser crepes can be formed in the sheet material as a whole; a desired degree of disruption of bonds between constituent fibers of a sheet material can occur; and softening, drapability and extensibility can be produced or enhanced. Heat-setting is typically employed when the treatment is of web materials having a thermoplastic component.
In such ways, the traveling flexible sheet materials can be softened or rendered permanently elastic; their appearance and feel can be made more like cloth; absorptive qualities of sheet materials can be improved; sheet materials can be given an improved ability to drape or conform about objects; and other useful qualities can be imparted.
Such microcreping is useful with a wide range of materials. For instance: nonwoven sheet materials comprised of natural fibers, synthetic fibers, or blends of the two kinds of fibers in single or multiple layers can be microcreped; plastic films or thicker plastic sheets, and nonwoven or fibrous sheets having a plastic film or metal coating or lamination can be microcreped; paper sheet materials and other sheet products produced from pulp can be microcreped, etc.
The practical development of the one roll microcreper (dry microcreper) traces back to Richard R. Walton and his associates. For instance U.S. Pat. No. 3,260,778, issued Jul. 12, 1966, describes a bladed one-roll microcreper. A material-confining retarder passage is defined between an angled blade-form retarder on one side and a flexible retarder member on the other side of the material. The treated material is forced to move outwardly from between these retarders in an extruding action while continuously, freely slipping past the retarder surfaces. U.S. Pat. No. 3,810,280, issued May 14, 1974, describes a bladeless one-roll microcreper that defines its retarder passage between the drive roll surface and an over-lying stationary retarder member which, rather than allowing the material to freely slip, engages and aggressively retards the material by a mechanical surface retarding effect (as opposed to retarding by confining the material to extrude between freely slippable surfaces, obtained with the bladed microcreper). Over the years, many variations of the one roll microcreper have been developed. A comb roll microcreper is shown in U.S. Pat. No. 4,090,385, issued May 23, 1978 and a bladed microcreper employing tangential extrusion is shown in U.S. Pat. No. 4,894,196, issued Jan. 16, 1990. Efforts to improve the system have continued over many years. For instance U.S. Pat. Nos. 4,717,329, issued Jan. 5, 1988 and 5,060,349, issued Oct. 29, 1991, relate to a replaceable pre-assembled system of the stationary members of a microcreper and U.S. Pat. No. 5,666,703, issued Sep. 16, 1997 and U.S. Pat. No. 5,678,288, issued Oct. 21, 1997, relate to improvements for bladeless microcrepers. Each of these patents is referred to, and in jurisdictions where it is possible, each is hereby incorporated by reference, to illustrate the decades-long effort to improve microcrepers and the wide variety of one roll microcreper arrangements that are possible.
During their long development the one roll microcreper (dry microcreper) treatments were found to be very sensitive to geometric and other variables. In particular it was determined to be vitally important to employ machine elements that are stable and uniform over time in the width and length dimensions of the machine. Bending or buckling, warping or puckering, lengthwise displacement or other geometrical variation of the stationary surfaces engaging the material in the critical driving and retarding regions could not be tolerated.
In this respect, one of the basic findings for the one roll microcreper was the necessity to use a stationary hard metal member such as spring steel as the contact or “primary” member to press the web material against the driven roll to drive the sheet material forward. The surface of the primary member was formed by a low friction, heat-resistant coating applied to the metal member, typically DuPont's Teflon, with the strength and dimensional stability of the metal being relied upon to maintain the working surface within critical geometric tolerances. This primary member was securely held so that a narrow area of its face could be pressed with controlled pressure into freely slippable relation upon the outer face of the flexible sheet material. This pressed the inner face of the material against the gripping surface of the moving roll surface. The resulting strong engagement with the roll surface enabled the flexible sheet material to be positively, mechanically driven forward in its plane, the flexible sheet material slipping forward under the stationary primary member in a continual motion, i.e., freely, without alternate slipping and stopping. By the stationary primary member being principally of metal, it was found that the primary member could be mechanically stable, i.e. without bending or buckling that would introduce non-uniformities to the treatment.
Similarly, in the case of bladed microcreper arrangements, it was also found that the stationary retarder members should likewise be formed of steel or other metal with similar properties.
By observing these conditions, for numerous sheet materials it was found that an acceptable balance was attainable between practical driving and retarding components, speed of operation, heating, wear-rate of the components and the need for a constant geometry of the treatment region across the width and throughout the length of the traveling material. But it also was found that there were significant limitations on use of the process. At desired production high rates, it was found that friction-generated heat at the stationary, freely slippable surfaces could harm many kinds of flexible sheet materials or cause heat distortion of the parts forming the drive and retarding region to disrupt the uniformity of the treatment. When treating many kinds of materials, the stationary slippable surfaces suffered undue wear. Because of such problems as overheating and undue wear, significant limitation on commercial use was thought to be inherent with respect to the kinds of materials that could be treated, the kinds of treatments that could be obtained, and the maximum speed of processing. In many cases, such production problems have made microcreping costly, in other cases microcreping seemed totally impractical.
As an example, many web materials of polymer fibers could not be microcreped commercially for desired end effects because, at commercially acceptable speed, frictional heating of the polymer to high local temperatures produced an excessively deformed or melted polymer state in those regions. For instance, this produced sharp-edges on undulations of the material at the surfaces of the material, providing a harsh sensation to the touch. This has especially been the case for nonwoven material of polyolefin fibers such as polypropylene or polyethylene, which are low cost and widely preferred for the manufacture of disposable diapers, personal care products, etc. Likewise, microcreping plain films and laminates that include films of polypropylene or polyethylene produce sharp and abrasive crepe edges at the surface due to polymer melting that are not acceptable in many cases.
As another example, microcreping of sheet material formed of wood pulp has been limited because of destruction of the stationary primary surface when the process is operated at desired high speeds. This has been the case for products produced of wood pulp such as Kraft papers and for nonwoven wipe products that have a high wood pulp content. In attempting microcreping of products formed of recycled wood pulp that contain abrasive fines, the primary member, i.e. its low friction coating, and soon, the underlying steel surface itself, has been ruined over a brief period of operation.
Other difficulties have arisen with microcreping due to the tendency of migratory substances such as inks to transfer from the materials being treated, producing accumulation of adherent deposits on the treatment surface that disrupt the treatment and involve costly down-time to remove. Another problem has been in respect of barrier coatings in which the process seemed to inherently produce pin holes in the barrier layer.